Large power stations with power range above 100 MW, in which a generator which produces electricity is driven by a gas turbine and/or steam turbine and the electrical power that is produced is fed into an electrical grid at a predetermined grid frequency (for example 50 or 60 Hz), normally have a fixed coupling between the (mechanical) rotation speed of the turbine and the grid frequency. The output of the generator is in this case connected via a grid connection at a locked frequency to the electrical grid, while it is driven either directly (single shaft installation) by the turbine, or with a coupled rotation speed via a mechanical gearbox. Configurations of power stations such as these are shown in a highly simplified form in FIGS. 2 and 3. Only fixed conversion ratios can be achieved between the grid frequency and the turbine by means of a gearbox. However, solutions are also feasible in which the generator is driven by a power turbine which can be driven at a different rotation speed to that of the actual gas turbine.
FIG. 2 shows a highly simplified illustration of a power station 10′ of a known type which produces electricity by means of a gas turbine 12 with a coupled generator 18, and feeds this into an electrical grid 21. The gas turbine 12 and the generator 18 are connected by a common shaft 19, and form a single-shaft turbine shafting 11. In the simplest case, the gas turbine comprises a compressor 13 which inducts and compresses combustion air via an air inlet 16. The compressor 13 may be formed from a plurality of partial compressors connected one behind the other, which operate at an increasing pressure level and may possibly allow intermediate cooling of the compressed air. The combustion air which is compressed in the compressor 13 passes into a combustion chamber 15, into which liquid fuel (for example oil) or gaseous fuel (for example natural gas) is injected via a fuel supply 17 and is burned, with combustion air being consumed.
The hot gases which emerge from the combustion chamber 15 are expanded in a downstream turbine 14 with work being carried out, and thus drive the compressor 13 and the coupled generator 18. The exhaust gas, which is still relatively hot when it leaves the turbine, can additionally be passed through a downstream heat recovery steam generator 23 in order to produce steam for operation of a steam turbine 24, in a separate water/steam cycle 25. A combination such as this is referred to as a combination power station. In this case, the steam turbine 24 may be coupled to the generator 18 on the opposite side of the turbine 14. However, it may also drive its own generator.
In the case of the single-shaft installation shown in FIG. 2, the rotation speed of the gas turbine 12 has a fixed ratio with respect to the frequency of the AC voltage as produced in the generator 18, which must be the same as the grid frequency of the electrical grid 21. With the large gas turbine units that are normal nowadays with powers of more than 100 MW, the generator frequency or grid frequency of 60 Hz is associated with a gas-turbine rotation speed of 3600 rpm (for example Model GT24 gas turbine by the assignee of the present application), and the generator frequency of 50 Hz is associated with a rotation speed of 3000 rpm (for example Model GT26 gas turbine also by the Assignee of the present application).
If it is intended to achieve a different ratio between the rotation speed of the gas turbine 12 and the generator or grid frequency, a mechanical gearbox 26 can in principle be inserted between the shaft 19 of the gas turbine 12 and the generator 18 (turbine shafting 11′) in a power station 10″ as shown in FIG. 3, which mechanical gearbox 26 is normally in the form of a reduction gearbox and therefore allows higher rotation speeds and smaller designs of the gas turbine 12. However, mechanical gearboxes 26 such as these can be used only for power levels below 100 MW, for strength reasons. On the other hand, the large power levels per gas turbine of more than 100 MW and the high efficiencies are achieved in particular with comparatively slowly rotating single-shaft machines.
This then results in the situation shown in FIG. 1. At a rating above 100 MW there are individual single-shaft gas turbines which are designed and optimized for a fixed rotation speed of either 3000 rpm (for 50 Hz; GT26) or 3600 rpm (for 60 Hz; GT24) (F. Joos et al., “Field Experience With the Sequential Combustion System of the GT24/GT26 Gas Turbine Family”, ABB Review no. 5, p. 12-20 (1998)). Above 100 Hz and for powers below 100 MW, virtually any desired AC voltage frequencies are possible (shaded area in FIG. 1) by configurations with a power turbine or gearbox, or by multiple shaft gas turbines. In this case, the powers of the gas turbines plotted against the frequency follow a curve A, while the efficiency η follows the curve B. High powers with high efficiencies can therefore be achieved in particular at low rotation speeds, although only singular solutions are available there.
In order to reduce the production costs for singular solutions, U.S. Pat. No. 5,520,512 proposes that at least parts of the turbines be designed to be identical for gas turbine installations for different grid frequencies. However, the rigid coupling between the rotation speed of the gas turbine and the grid frequency remains unchanged in this case.
U.S. Pat. No. 6,628,005 proposes that a single-shaft installation comprising the turbine and generator with the predetermined rotation speed be made usable for different grid frequencies of 50 Hz and 60 Hz by choosing a generator frequency between the two grid frequencies, for example 55 Hz, and by adding or subtracting 5 Hz, by a frequency differentiator, depending on the grid frequency. A rigid coupling is still maintained in this case as well.
The following disadvantages result from the rigid coupling between the turbine rotation speed and the grid frequency for existing installation concepts with existing turbo components:                A stable operation on the electrical grid is possible only to a restricted extent        Power level dips occur in the turbine, and excessive thermal and mechanical loads in the case of dynamic control for grid frequency support by raising the gas turbine inlet temperature.        Rapid transients lead to increased loads.        It is impossible to control the power of the power station independently of the grid frequency.        It is impossible to optimize the efficiency of the power station independently of the grid frequency.        It is impossible to optimize partial load of the power station independently of the grid frequency.        Emission control for the gas turbine is possible only to a restricted extent.        
The following disadvantages result from the rigid coupling between the turbine rotation speed and the grid frequency for existing installation concepts, with components that need to be newly developed or else new installations:                Compressors and turbines for fixed frequency coupling cannot be designed for an optimum point as will be possible without frequency dependency.        Gas turbines and steam turbines which are designed for fixed 50 Hz or 60 Hz grid frequency coupling are not necessarily cost-optimum for a desired power level since, as a result of the predetermined rotation speed, aerodynamic or mechanical design limits impede optimization, and these limits can be better matched to one another if the rotation speed is variable.        The power level of power station turbines is limited by the predetermined coupling to the grid frequency (see curve A in FIG. 1).        The gas turbines cannot be optimally matched to variable environmental conditions.        
U.S. Pat. No. 5,694,026 discloses a single-shaft turbogenerator set without step-down gearbox, in which a static frequency converter is arranged between the output of the generator and the electrical grid, with the aid of which the AC voltage frequency produced by the generator is converted to the grid frequency. When the gas turbine is started, the generator is used as a motor and is supplied with power via the static frequency converter from the electrical grid. The converter contains a direct-current intermediate circuit formed from an inductance.
U.S. Pat. No. 6,979,914 discloses a power station having a single-shaft arrangement comprising a gas turbine and generator, in which a converter is likewise provided between the generator output and the electrical grid, in order to match the AC voltage produced by the generator to the grid frequency. A DC voltage intermediate circuit is in this case arranged in the converter.
A power station having a high-speed-rotating gas turbine (18,000 rpm) and a comparatively low output power (1600 kW) is known from the article by L. J. J. Offringa, et al. “A 1600 kW IGBT Converter With Interphase Transformer for High Speed Gas Turbine Power Plants”, Proc. IEEE—IAS Conf. 2000, 4, 8-12 Oct. 2000, Rome, 2000, pages 2243-2248, in which frequency decoupling between the generator and the electrical grid is achieved by a converter with a DC voltage intermediate circuit.
Known power stations with decoupling between the generator output and the electrical grid by a frequency converter with a direct-current or DC voltage intermediate circuit have the disadvantage that the converters result in not inconsiderable power losses which, in the case of power stations with a single-shaft turbine section and powers of more than 100 MW, partially counteract the efficiency improvement achieved in this area, again.